Recent results in the development of GaN-based light-emitting diodes (LEDs) and laser diodes LDs) operating in the green, blue, and ultraviolet spectrum have demonstrated the tremendous scientific and commercial potential of group III nitride semiconductors (e.g., GaN, AlN, InN, and their alloys). These applications require electrically conducting substrates (e.g., GaN or AlGaN) so that a vertical device geometry can be utilized in which the electrodes are located on top and bottom surfaces of the device structure. In addition to opto-electronic devices, group III nitride semiconductors can be used in a host of other applications such as communication electronics (e.g., high power microwave devices). These devices require electrically insulating substrates.
In order to achieve the desired device performance (e.g., high efficiency, low leakage current, high device yield, long lifetime, etc.) for devices fabricated from group III nitride semiconductor materials, it is expected that such devices will have to be fabricated on native GaN or AlGaN substrates. Native substrates as used herein refers to substrates that have been obtained from bulk material, the bulk material preferably grown from seeds of the same composition, thus allowing the substrates to achieve extremely low defect densities as well as low residual stress levels. Unfortunately, due to the lack of bulk GaN and AlGaN substrates, device developers have been forced to attempt various work-around solutions.
Initially GaN-based structures were developed on foreign substrates. Some of the substrates that have been used in these attempts are sapphire, silicon carbide, ZnO, LiGaO2, LiAlO2, and ScMgAlO4. These devices suffer from a high defect density including a high density of dislocations, domains, and grain boundaries. Additionally, these devices suffer from cracking and bending of the epitaxial structures. Most of these problems arise from the lattice and thermal mismatch between the foreign substrates and the GaN-based device structures. As a result, these devices exhibit greatly reduced performance.
In order to attempt to overcome the problems in growing a GaN-based device on a foreign (i.e., non-GaN) substrate, a number of developers have attempted to grow GaN single crystals suitable for use with microelectronic device structures. For example, U.S. Pat. No. 5,679,152 discloses a technique for growing a GaN layer on a sacrificial substrate and then etching away the substrate. As disclosed, the substrate is etched away in situ while the substrate/GaN layer is at or near the growth temperature. The GaN layer may be deposited directly on the sacrificial substrate or deposited on an intermediate layer that has been deposited on the substrate.
Alternately, U.S. Pat. No. 5,770,887 discloses a repetitive growth technique utilizing alternating epitaxially grown layers of a buffer material and GaN single crystal. Removing the buffer layers, for example by etching, produces a series of GaN single crystal wafers. The patent discloses using sapphire as the initial seed crystal and a material such as BeO, MgO, CaO, ZnO, SrO, CdO, BaO or HgO as the buffer layer material.
Another technique for growing GaN substrates is disclosed in U.S. Pat. No. 6,146,457. As disclosed, a thick layer of GaN is epitaxially deposited on a thin, disposable substrate using a CVD process. In the preferred embodiment, the substrate (e.g., sapphire) has a thickness of 20-100 microns while the GaN epitaxial layer has a thickness of 50-300 microns. Accordingly, upon material cooling, the strain arising from the thermal mismatch of the material is relieved by cracking the disposable substrate rather than the newly deposited epitaxial layer. As noted by the inventors, however, the disclosed process solves the problems associated with thermal mismatch, not lattice mismatch.
U.S. Pat. No. 6,177,292 discloses a method for growing a GaN group material on an oxide substrate without cracking. After the growth of the GaN group material, a portion of the oxide substrate is removed by mechanical polishing. Another GaN layer is then grown on the initial GaN layer to provide further support prior to the complete removal of the remaining oxide substrate. As a result of this process, a stand-alone GaN substrate is formed that can be used as a substrate for the growth of a micro-electronic device.
U.S. Pat. No. 6,218,280 discloses a method and apparatus for depositing III-V compounds that can alternate between MOVPE and HVPE processes, thus not requiring that the substrate be cooled and transported between the reactor apparatus during device growth. As disclosed, the MOVPE process is used to grow a thin III-V nitride compound semiconductor layer (e.g., a GaN layer) on an oxide substrate (e.g., LiGaO2, LiAlO2, MgAlScO4, Al2MgO4, and LiNdO2). The HVPE process is then used to grow the device structure on the GaN layer.
The main problems associated with growing true bulk GaN or AlGaN crystals relate to fundamental properties of the materials. For example, sublimation growth of GaN is limited by the incongruent decomposition of GaN that becomes noticeable at temperatures from 800° C. to 1100° C. In U.S. Pat. No. 6,136,093, a technique is disclosed for growing GaN on a GaN seed. As disclosed, Ga is heated to or above the evaporation temperature of Ga and the seed crystal is heated to a temperature higher than that of the Ga. The Ga vapor then reacts with a nitrogen containing gas to form GaN which is deposited on the seed crystal.
Another method to grow bulk GaN crystals is to grow them from the liquid phase. The main problem associated with liquid phase growth of GaN is the extremely low solubility of nitrogen in melts in general, and in Ga melts in particular. The low solubility limits the GaN growth rate and results in small volume GaN crystals, generally less than 0.2 cubic centimeters.
In order to overcome the low solubility of nitrogen in the Ga melts, typically growth temperatures between 1500° C. and 1600° C. are used with a nitrogen gas pressure in the range of 10 to 20 kilobars. Even at these high pressures and temperatures, nitrogen solubility in the Ga melt is very low. As a result, only growth rates of approximately 0.01 to 0.05 millimeters per hour can be obtained. Lateral growth rate, i.e., the growth rate perpendicular to the [0001] crystallographic direction, is typically about 1 millimeter per day. In addition to the low growth rates, due to the high temperatures and pressures it is difficult to develop production techniques using this process.
Accordingly, although III-V nitride compound semiconductor materials offer tremendous potential for a variety of micro-electronic devices ranging from opto-electronic devices to high-power, high-frequency devices, the performance of such devices is limited by the lack of suitable substrates. The present invention provides a method for fabricating suitable substrates.